This invention pertains generally to directional couplers used in microwave circuitry, and particularly to directional couplers fabricated in suspended stripline and adapted for use over extremely broad frequency bands.
It is known in the art that directional couplers fabricated in stripline (sometimes referred to hereinafter as "stripline couplers") may be used in many applications in microwave circuitry. For example, when low loss and constant dielectric properties are of paramount importance, a so-called suspended stripline coupler may be used to advantage. Such a coupler differs from a conventional stripline coupler in that a suspended stripline coupler comprises a printed circuit of appropriate shape supported (without a solid spacer having a dielectric constant greater than unity) between two opposing ground planes. The absence of a solid dielectric material reduces, as compared with a conventional stripline coupler, the susceptibleness of a suspended stripline coupler to changes in insertion loss and directivity when the frequency of an impressed microwave signal is changed.
unfortunately, a suspended stripline coupler fabricated in any known manner is satisfactorily operable on microwave energy only at a frequency that is within a relatively narrow band of frequencies encompassing a design frequency. However, many applications require satisfactory operation at any frequency within a relatively wide band of frequencies. For example, in a monopulse radar incorporating frequency diversity as a palliative against electronic countermeasures, it is necessary that a directional coupler operate satisfactorily over an extremely wide frequency band. Satisfactory operation, i.e., flat coupling over a broad frequency range, requires a multi-section coupler in which "even" and "odd" mode propagation of microwave energy occurs. An "even" mode of propagation means that an effective open circuit appears between the main and branch lines of a directional coupler. An "odd" mode means that an effective short circuit appears between the main and branch lines of a directional coupler. Because the cross section of supended stripline is not homogeneous, the phase velocity of propagation in the odd mode is less than that in the even mode, with the result that flat coupling and high directivity of known multi-section couplers may not be satisfactorily achieved throughout a wide band of frequencies. With the foregoing background in mind, it is a primary object of this invention to provide a directional coupler in stripline, such coupler being operable over a wide band of freqencies.
Another object of this invention is to provide a multisection directional coupler in which the phase velocities of microwave energy propagated in either the odd or even mode of propagation are substantially the same throughout a wide band of operating frequencies.
The primary object of this invention, and others that will become evident, are attained generally by providing in a four-port suspended stripline coupler adapted to divide microwave power applied at a first port into unequal amounts at a second and third port, with substantially no power at a fourth port, such coupler being characterized by a matching arrangement to match the phase velocities within such coupler of microwave energy propagating in the odd and even modes.